In magnetic resonance measurements, the interaction of magnetic torques of atomic nuclei, the nuclear spins, is examined using an external magnetic field. The nuclear spins align themselves in the external magnetic field and process with a Larmor frequency. The Larmor frequency depends on the value of the magnetic torque of the atomic nucleus when excited by an external electromagnetic alternating field about the axis of orientation in the magnetic field. The atomic nuclei generate an electromagnetic alternating field with the Larmor frequency.
The external electromagnetic alternating field for exciting the nuclear spins is irradiated by one or more antennas into a sample or into a patient. One possible form of antenna is a body coil, which surrounds the patient or the sample. Local coils are however also used, which may be arranged directly on the patient or the sample. The electromagnetic alternating field generated by the atomic nuclei is also received by the antennas. The same antenna may receive the signal generated, or the nuclear spins may be excited using one type of antenna and the electromagnetic alternating field generated by the atomic nuclei is received by another type of antenna.
Both during the generation of the high-frequency electromagnetic alternating fields and also during the actuation of the gradient coils for the spatial resolution, powers in the range of kilowatts and voltages of several hundred or thousand volts are used. Hence the actuations also generate power losses of a comparable magnitude and benefit from efficient and high-capacity cooling. Also, because the measured high-frequency signals resulting from the excited nuclear spins are very weak, it is useful to ensure that electromagnetic interference fields generated by the actuations do not unintentionally bleed from the actuations and cause the measured signals to degrade.